Description
2.6 Protection system
3VA molded case circuit breakers with IEC certificate
Manual, 03/2019, A5E03603177010-03
75
The ID letter for overload protection is L (stands for "Long-time delay").
The trip unit is inverse-time delayed and exhibits the following characteristics depending on
the trip unit type:
● Bimetal characteristic with thermal-magnetic trip units
● I
2
t characteristic with electronic trip units
The letters I
r
refer to the current setting value, and t
r
to the associated time delay.
The time delay with the device-specific reference point defines the current-dependent curve
shape of the setpoint characteristic. With 3VA molded case circuit breakers, this trip unit
reference point corresponds to 6 times the current setting value I
r
. The following
mathematical equation determines the remaining curve of the setpoint characteristic:
I
2
t = constant
The electronic trip units feature a thermal memory function that can be deactivated on some
electronic overload releases.
Thermal memory activated (ON)
If the thermal memory is activated, the previous thermal history is taken into account. After
tripping, the thermal memories of the phases are set to a value corresponding to 85% of the
thermal equivalent of the warmest phase. This setting allows reclosure of the contacts of the
molded case circuit breaker. This is followed by a cool-down period according to an
exponential function with a time constant T
au
(τ).
Thermal memory deactivated (OFF)
It is the responsibility of the commissioning engineer/operator to provide additional thermal
overload protection for the lower-level power distribution system while the thermal memory is
deactivated.
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